Aln crystal preparation method, aln crystals, and organic compound including aln crystals

ABSTRACT

An AlN crystal preparation method includes using at least one element excluding Si that fulfills the condition that a compound is not formed with either Al or N or the condition that a compound is formed with either Al or N but the standard free energy of formation of said compound is greater than the standard free energy of formation of AlN. In the preparation method, a composition including at least Al and the element is melted. Al vapor and nitrogen gas are reacted at a prescribed reaction temperature. AlN crystals are formed.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This is a National Stage Entry into the United States Patent andTrademark Office from International PCT Patent Application No.PCT/JP2015/064605, having an international filing date of May 21, 2015,which claims priority to both Japanese Patent Application No.2014-112691, filed May 30, 2014, and Japanese Patent Application No.2014-181289, filed Sep. 5, 2014, the entire contents of all of which areincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a method for producing AlN crystals,AlN crystals and an organic compound comprising AlN crystals.

DESCRIPTION OF RELATED ART

AlN whiskers (needle- or rod-shaped single crystals), which are one typeof AlN (aluminum nitride) crystal, have the properties of beingexcellent in thermal conductivity and insulation. As for a method forproducing the AlN whiskers, Japanese Patent Application Publication No.62-283900 (“the '900 Publication”) discloses a method which involvesmixing a fine alumina powder with a solution of a transition metalcompound, then evaporatively removing the solvent by heat treatment,homogeneously mixing carbon black with the heat-treated fine aluminapowder to prepare a mixture, and reacting the prepared mixture withnitrogen gas at a predetermined reaction temperature. Also, JapanesePatent Application Publication No. 2014-73951 (“the '951 Publication”)discloses a method which involves using an alloy having Al—Si—Ticomposition, melting the alloy, and reacting the Al vapor with nitrogengas at a predetermined reaction temperature.

SUMMARY OF THE INVENTION

A problem of the method of the '900 Publication is a poor yield(production efficiency) of AlN whiskers because particulate AlN isproduced in addition to the AlN whiskers. The method of the '951Publication can promote the growth of AlN whiskers and suppress theproduction of particulate AlN. However, an element that constitutes thecomposition with Al is limited to Si and Ti. Therefore, a problem ofthis method is a restricted condition for producing AlN whiskers.

The present invention has been made in light of these situations, and anobject of the present invention is to provide a method for producing AlNcrystals which can produce the AlN crystals by using an element otherthan Si and Ti while suppressing the production of particulate AlN, AlNcrystals and an organic compound comprising AlN crystals.

The method for producing AlN crystals comprises: using at least oneelement, excluding Si, that satisfies a condition under which theelement forms a compound with neither Al nor N or a condition underwhich the element forms a compound with any of Al and N provided thatthe standard free energy of formation of the compound is larger thanthat of AlN; melting a composition containing at least Al and theelement; and reacting the Al vapor with nitrogen gas at a predeterminedreaction temperature to produce AlN crystals.

The method for producing AlN crystals comprises using, as the element,an element that satisfies a condition under which the interaction energywith Al becomes negative and also satisfies a condition under which theabsolute value of this interaction energy is larger than the interactionenergy between Al and Ge.

According to the method for producing AlN crystals, at least oneelement, excluding Si, that satisfies a condition under which theelement forms a compound with neither Al nor N or a condition underwhich the element forms a compound with any of Al and N provided thatthe standard free energy of formation of the compound is larger thanthat of AlN is used as an element that constitutes the alloy with Al.Furthermore, a composition containing at least Al and the element ismelted, and the Al vapor is reacted with nitrogen gas at a predeterminedreaction temperature to produce AlN crystals.

Use of at least one element that satisfies a condition under which theelement forms a compound with neither Al nor N or a condition underwhich the element forms a compound with any of Al and N provided thatthe standard free energy of formation of the compound is larger thanthat of AlN can promote the growth of the AlN crystals while suppressingthe production of particulate AlN, because neither a compound of Al andthe element nor a compound of N and the element is formed or because thestandard free energy of formation of such a compound, even if formed, islarger than that of AlN (the compound is thermodynamically more unstablethan AlN). As a result, the element that constitutes the compositionwith Al for the AlN crystal production is not limited to Si and Ti, andthe AlN crystals can be produced using an element other than Si and Ti.

The AlN crystals are allowed to grow from a solution according to asolution method by melting the composition containing at least Al andthe element. Therefore, the grown AlN crystals can achieve high thermalconductivity and high insulation without containing fine particles. Inthe case of allowing AlN whiskers to grow as the AlN crystals, forexample, the method disclosed in Japanese Patent Application PublicationNo. 6-321511 or Japanese Patent Application Publication No. 7-144920involves forming fine particles by heat-melting the alloy with arccurrent so that the shape of the fine particles becomes a needle or rodshape to produce the AlN whiskers. This method, however, cannot avoidthe inclusion (coexistence) of fine particles that have not assumed aneedle or rod shape. As a result, the fine particles are contained inthe AlN whiskers. By contrast, the method, as in the present invention,which involves allowing the AlN crystals to grow from a solutionaccording to a solution method, can allow the AlN crystals to growwithout containing such fine particles.

The method for producing AlN crystals employs, as the element, anelement that satisfies a condition under which the interaction energywith Al becomes negative and also satisfies a condition under which theabsolute value of this interaction energy is larger than the interactionenergy between Al and Ge. The element satisfying the condition underwhich the interaction energy with Al becomes negative has a propensityto be easily adsorbed onto Al on the AlN surface. When the AlN crystalsare AlN whiskers, the AlN whisker grown in the <0001> direction has twoend faces, one of which is covered with N (N-face) and the other ofwhich is covered with Al (Al-face). It is generally known about thegrowth of AlN by a sublimation method that the stable growth of the AlNcrystal is promoted at the N-face, whereas the growth of the AlN crystalis inhibited at the Al-face, for example, due to the formation of facetssuch as (10-11). In short, when the AlN crystals are AlN whiskers, thegrowth of the AlN whisker at the Al-face is inhibited when the elementsatisfying the condition under which the interaction energy with Albecomes negative is adsorbed on the Al-face of the AlN whisker. As aresult, the growth of the AlN whisker at the N-face, which permitsstable growth, can be promoted so that the AlN whisker grows stablytoward the direction of the N-face to produce AlN whiskers having flatsurface (surface with very few irregularities) in the axial direction(growth direction).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows one embodiment of the present invention and is a diagramshowing an AlN whisker production apparatus (part 1).

FIG. 2 is a diagram showing an AlN whisker production apparatus (part2).

FIG. 3 is a diagram showing an AlN whisker production apparatus (part3).

FIG. 4 is a diagram showing the presence or absence of compoundformation with Al and the presence or absence of compound formation withN (part 1).

FIG. 5 is a diagram showing the presence or absence of compoundformation with Al and the presence or absence of compound formation withN (part 2).

FIG. 6 is a diagram showing the presence or absence of compoundformation with Al and the presence or absence of compound formation withN (part 3).

FIG. 7 is a diagram showing the presence or absence of compoundformation with Al and the presence or absence of compound formation withN (part 4).

FIG. 8 is a diagram showing Al—X interaction energy.

FIG. 9 is a diagram showing steps for producing an Al—Fe alloy.

FIG. 10 is a diagram showing steps for producing AlN whiskers.

FIG. 11 is a composition diagram of Al—Fe.

FIG. 12 is a diagram showing a temperature gradient.

FIG. 13 is a diagram schematically showing the growth of AlN whiskers.

FIG. 14 is a diagram showing the relationship between the vapor pressureof Al and the vapor pressure of nitrogen gas.

FIG. 15 is a diagram showing an image of a photographed Al—Fe alloy(part 1).

FIG. 16 is a diagram showing an image of photographed AlN whiskers (part1).

FIG. 17 is a diagram showing an image of photographed AlN whiskers (part2).

FIG. 18 is a diagram showing an image taken with a scanning electronmicroscope (part 1).

FIG. 19 is a diagram showing an image taken with a scanning electronmicroscope (part 2).

FIG. 20 is a diagram showing an image taken with a scanning electronmicroscope (part 3).

FIG. 21 is a diagram showing an image taken with a scanning electronmicroscope (part 4).

FIG. 22 is a diagram showing an image taken with a scanning electronmicroscope (part 5).

FIG. 23 is a diagram showing an image taken with a scanning electronmicroscope (part 6).

FIG. 24 is a diagram showing analysis results of XRD.

FIG. 25 is a diagram showing an image taken with a transmission electronmicroscope (part 1).

FIG. 26 is a diagram showing results of analyzing mass concentration andatomic concentration (part 1).

FIG. 27 is a diagram showing spectra (part 1).

FIG. 28 is a diagram showing an image taken with a transmission electronmicroscope (part 2).

FIG. 29 is a diagram showing results of analyzing mass concentration andatomic concentration (part 2).

FIG. 30 is a diagram showing spectra (part 2).

FIG. 31 is a diagram showing an image taken with a transmission electronmicroscope (part 3).

FIG. 32 is a diagram showing a mapping image of Al.

FIG. 33 is a diagram showing a mapping image of N.

FIG. 34 is a diagram showing a mapping image of O.

FIG. 35 is a diagram showing a mapping image of O.

FIG. 36 is a diagram showing an image taken with a transmission electronmicroscope (part 4).

FIG. 37 is a diagram showing an image taken with a transmission electronmicroscope (part 5).

FIG. 38 is a diagram showing an image taken with a scanning electronmicroscope (part 7).

FIG. 39 is a diagram showing an image taken with a transmission electronmicroscope (part 6).

FIG. 40 is a diagram showing an image taken with a transmission electronmicroscope (part 7).

FIG. 41 is a diagram showing an image of a photographed Al—Fe alloy(part 2).

FIG. 42 is a diagram showing an image of photographed AlN whiskers (part3).

FIG. 43 is a diagram showing an image of photographed AlN whiskers (part4).

FIG. 44 is a diagram showing an image of photographed AlN whiskers (part5).

FIG. 45 is a diagram showing an image taken with a scanning electronmicroscope (part 8).

FIG. 46 is a diagram showing an image taken with a scanning electronmicroscope (part 9).

FIG. 47 is a diagram showing an image taken with a transmission electronmicroscope (part 8).

FIG. 48 is a diagram showing an image taken with a transmission electronmicroscope (part 9).

FIG. 49 is a diagram showing an image of photographed AlN whiskers (part6).

FIG. 50 is a diagram showing an image taken with a scanning electronmicroscope (part 10).

FIG. 51 is a diagram showing an image taken with a scanning electronmicroscope (part 11).

FIG. 52 is a diagram showing an image taken with a scanning electronmicroscope (part 12).

FIG. 53 is a diagram showing an image of photographed AlN whiskers (part7).

FIG. 54 is a diagram showing an image taken with a scanning electronmicroscope (part 13).

FIG. 55 is a diagram showing an image taken with a scanning electronmicroscope (part 14).

FIG. 56 is a diagram showing an image of photographed AlN whiskers (part8).

FIG. 57 is a diagram showing an image taken with a scanning electronmicroscope (part 15).

FIG. 58 is a diagram showing an image taken with a scanning electronmicroscope (part 16).

FIG. 59 is a diagram showing an image of photographed AlN whiskers (part9).

FIG. 60 is a diagram showing an image taken with a scanning electronmicroscope (part 17).

FIG. 61 is a diagram showing an image taken with a scanning electronmicroscope (part 18).

FIG. 62 is a diagram showing an image taken with a scanning electronmicroscope (part 19).

FIG. 63 is a diagram showing an image of photographed metals of Al—Co.

FIG. 64 is a diagram showing an image of photographed AlN whiskers (part10).

FIG. 65 is a diagram showing an image taken with a scanning electronmicroscope (part 20).

FIG. 66 is a diagram showing an image taken with a scanning electronmicroscope (part 21).

FIG. 67 is a diagram showing an image taken with a scanning electronmicroscope (part 22).

FIG. 68 is a diagram showing an image taken with a scanning electronmicroscope (part 23).

FIG. 69 is a diagram showing an image taken with a scanning electronmicroscope (part 24).

FIG. 70 is a diagram showing an image of photographed metals of Al—Ge.

FIG. 71 is a diagram showing an image of photographed AlN whiskers (part11).

FIG. 72 is a diagram showing an image taken with a scanning electronmicroscope (part 25).

FIG. 73 is a diagram showing an image taken with a scanning electronmicroscope (part 26).

FIG. 74 is a diagram showing an image taken with a scanning electronmicroscope (part 27).

FIG. 75 is a diagram showing an image of photographed metals of Al—Sn.

FIG. 76 is a diagram showing an image of photographed AlN whiskers (part12).

FIG. 77 is a diagram showing an image taken with a scanning electronmicroscope (part 28).

FIG. 78 is a diagram showing the relationship between the amount of AlNwhiskers added to a resin and thermal conductivities.

FIG. 79 is a diagram showing the cross section of an electric wire.

FIG. 80 is a diagram showing the cross section of a light bulb.

FIG. 81 is a diagram showing the cross section of a motor.

FIG. 82 is a diagram showing the cross section of an electroniccomponent (part 1).

FIG. 83 is a diagram showing the cross section of an electroniccomponent (part 2).

FIG. 84 is a diagram showing the cross section of a printed circuitboard (part 1).

FIG. 85 is a diagram showing the cross section of a printed circuitboard (part 2).

FIG. 86 is a diagram showing the cross section of a sheet.

FIG. 87 is a diagram showing grease.

FIG. 88 is a diagram showing the cross section of paper.

FIG. 89 is a diagram showing the cross section of a nonwoven fabric.

FIG. 90 is a diagram showing the cross section of thread.

FIG. 91 is a diagram showing the cross section of twisted thread.

DETAILED DESCRIPTION OF EMBODIMENT(S) OF THE INVENTION

Hereinafter, a method for producing AlN whiskers, which is one type ofAlN crystal, through growth by a solution method according to oneembodiment of the present invention will be described with reference tothe drawings. The AlN whiskers are single crystals grown outward fromthe crystal surface in a needle or rod shape and are also called AlNwhisker fibers, AlN fibers or AlN fillers. As shown in FIG. 1, an AlNwhisker production apparatus 1 is a horizontal high-frequency heatingfurnace 2 and has a cylindrical quartz tube (core tube) 3 and ahigh-frequency coil (induction coil) 4 for heating which is disposedaround the quartz tube 3. A carbon heater 5 and an insulator 6 whichseparates the quartz tube 3 from the carbon heater 5 are disposed in theinside of the quartz tube 3. Alternatively, a SiC heater, a molybdenumdisilicide heater, a tungsten heater or the like may be used instead ofthe carbon heater 5.

A lid member 7 is attached on one end side of the quartz tube 3 (on theright side in FIG. 1). The lid member 7 is connected with a pipe 8 forintroducing argon (Ar) gas or nitrogen (N₂) gas to the inside of thequartz tube 3. A regulating valve 9 is provided at some midpoint of thepipe 8. The flow rate of argon gas or nitrogen gas to be introduced tothe inside of the quartz tube 3 is adjusted by adjusting the degree ofopening of the regulating valve 9. A tip 8 a of the pipe 8 is connectednear a site where an alumina board 14 mentioned later is disposed.

A lid member 10 is attached on the other end side of the quartz tube 3(on the left side in FIG. 1). The lid member 10 is connected with a pipe11 for discharging argon gas or nitrogen gas from the inside of thequartz tube 3 and also connected with a pipe 12 for vacuuming the insideof the quartz tube 3. A pump (rotary pump, diffusion pump or turbo pump)(not shown) for vacuuming is provided on the tip side of the pipe 12,while a regulating valve 13 is provided in at some midpoint thereof. Theinside of the quartz tube 3 is vacuumed by driving the pump and therebyadjusting the degree of opening of the regulating valve 13.

An alumina board (alumina container) 14 can be disposed on the carbonheater 5. The alumina board 14 is provided with a recess 14 a, and therecess 14 a is charged with a solvent 15. The solvent 15 is a liquid inwhich powders of Al and an element that satisfies conditions mentionedlater are dissolved. These components do not have to be in a powderstate. FIG. 1 illustrates a liquid containing Al and Fe powdersdissolved therein. FIG. 1 illustrates the case where the alumina board14 is disposed and the recess 14 a of the alumina board 14 is chargedwith the solvent 15. As shown in FIG. 2, an alloy 22 may be disposed onan alumina plate 21. The alloy 22 is a composition comprising Al and anelement that satisfies conditions mentioned later. FIG. 2 illustrates analloy having Al—Fe composition.

Alternatively, a configuration as shown in FIG. 3 may be used inconsideration of mass productivity. In FIG. 3, an AlN whisker productionapparatus 31 is a box high-frequency heating furnace 32. A heater 33 isdisposed in the inside of the high-frequency heating furnace 32. Also,an insulator (heat-resistant brick, carbon wool, etc.) 35 is disposedbetween the high-frequency heating furnace 32 and a furnace wall 34. Apipe 35 for introducing argon gas or nitrogen gas to the inside of thehigh-frequency heating furnace 32 is connected on one end side of thehigh-frequency heating furnace 32 (on the lower side in FIG. 3). Aregulating valve 36 is provided at some midpoint of the pipe 35. Theflow rate of argon gas or nitrogen gas to be introduced to the inside ofthe high-frequency heating furnace 32 is adjusted by adjusting thedegree of opening of the regulating valve 36. Also, a pipe 37 forvacuuming the inside of the high-frequency heating furnace 32 isconnected on one end side of the high-frequency heating furnace 32. Apump (not shown) for vacuuming is provided on the tip side of the pipe37, while a regulating valve 38 is provided at some midpoint thereof.The inside of the high-frequency heating furnace 32 is vacuumed bydriving the pump and thereby adjusting the degree of opening of theregulating valve 38. Pipes 39 and 40 for discharging argon gas ornitrogen gas from the inside of the high-frequency heating furnace 32are connected on the other end side 32 b of the high-frequency heatingfurnace 32 (on the upper side in FIG. 3). In this case, it is preferredthat the internal temperature of the high-frequency heating furnace 32should be uniformly controlled, while the argon gas or the nitrogen gasto be introduced to the inside of the high-frequency heating furnace 32should be uniformly controlled.

In the present embodiment, it is desirable that the element for use inthe solvent 15 or the alloy 22, i.e., the element that constitutes thecomposition with Al, should satisfy a condition (i) from the viewpointof producing the AlN whiskers:

(i) the element forms a compound with neither Al nor nitrogen gas, orthe element forms a compound with any of Al and nitrogen gas providedthat the standard free energy of formation of the compound is largerthan that of AlN. The standard free energy of formation is an energythat works when a substance (compound) is formed from a simplesubstance.

Specifically, the inventors examined whether or not some elements wouldeach form a compound with Al (the presence or absence of compoundformation) and would each form a compound with N, and calculated thestandard free energy of formation of the compound thus formed. FIG. 4shows results obtained at 1700 K. The standard free energy of formationof AlN at 1700 K is “−129.591 KJ/mol”, and Li, Mg, V, Cr, Mn, Fe, Co,Ni, Cu, Ga, Ge, Sr and Sn were determined as the element satisfying thecondition (i). FIG. 5 shows results obtained at 1800 K. The standardfree energy of formation of AlN at 1800 K is “−117.869 KJ/mol”, and Li,Mg, V, Cr, Mn, Fe, Co, Ni, Cu, Ga, Ge, Sr and Sn were determined as theelement satisfying the condition (i). FIG. 6 shows results obtained at1900 K. The standard free energy of formation of AlN at 1900 K is“−106.151 KJ/mol”, and Li, Mg, Si, V, Cr, Mn, Fe, Co, Ni, Cu, Ga, Ge, Srand Sn were determined as the element satisfying the condition (i). FIG.7 shows results obtained at 2000 K. The standard free energy offormation of AlN at 2000 K is “−94.436 KJ/mol”, and Li, Mg, Si, V, Cr,Mn, Fe, Co, Ni, Cu, Ga, Ge, Sr and Sn were determined as the elementsatisfying the condition (i). Si satisfies the condition (i) at 1900 Kand 2000 K, but does not satisfy the condition (i) at 1700 K and 1800 K.Accordingly, Li, Mg, V, Cr, Mn, Fe, Co, Ni, Cu, Ga, Ge, Sr and Sn weredetermined as the element satisfying the condition (i) at a reactiontemperature for AlN (1450 to 1800° C.).

In addition, it is desirable that the element should satisfy both of theconditions (i) and (ii) from the viewpoint of suppressing the surfaceirregularities of the AlN whiskers:

(ii) the interaction energy with Al is negative, and the absolute valueof this interaction energy is larger than the interaction energy betweenAl and Ge.

The interaction energy is an energy that works for the gravitationalinteraction between two atoms.

Specifically, Li, Cr, Fe, Co, Ni, Cu and Sr were determined as theelement satisfying both of the conditions (i) and (ii) on the basis ofthe interaction energy of (Al (30 at %)-X (70 at %)) in a solution at1700° C. shown in FIG. 8.

Next, a method for producing AlN whiskers by using an Al—Fe alloyobtained using Fe, which is an element satisfying both of the conditions(i) and (ii) described above, will be described. Here,

(1) steps for producing an Al—Fe alloy (see FIG. 9) and

(2) steps for producing AlN whiskers (see FIG. 10)

will be described in order.

(1) Steps for Producing Al—Fe Alloy

Step 1: In order to produce the alloy having a desired molar ratio, therespective weights of the elements (Al and Fe) are measured, and acrucible is charged with these elements. The crucible is a cruciblecomposed mainly of aluminum oxide or a crucible composed mainly ofaluminum oxide and magnesium oxide. Alternatively, a crucible composedmainly of carbon may be used. In the present embodiment, in the case ofproducing, for example, an alloy having Al (30 at %)-Fe (70 at %)composition, the weight of 30 parts by mol of Al (purity: 95% or higher,preferably 99% or higher) particles or plate and the weight of 70 partsby mol of Fe (purity: 95% or higher, preferably 99% or higher) particlesor plate are measured, and the crucible is charged with these elements.FIG. 11 shows the Al—Fe composition diagram.

Step 2: The crucible charged with Al and Fe is loaded in ahigh-frequency heating furnace, and the inside of the high-frequencyheating furnace is vacuumed. In this respect, the degree of vacuum isset to 1 mmHg or lower, preferably 1×10⁻¹ mmHg or lower.

Step 3: After the vacuuming of the inside of the high-frequency heatingfurnace, argon gas (purity: 95% or higher, preferably 99% or higher) isintroduced to the inside of the high-frequency heating furnace so thatthe internal pressure of the high-frequency heating furnace is adjustedto about atmospheric pressure. In this respect, it is preferred thatoxygen in the argon gas should be removed with a titanium getter, orwater vapor should be removed by liquefication in a condenser tubecooled with liquid nitrogen.

Step 4: Current is applied to the high-frequency coil to start theheating of the high-frequency heating furnace. The high-frequencyheating furnace is heated to a temperature of approximately 1700° C.over approximately 0.5 to 2 hours, kept at the temperature ofapproximately 1700° C. for approximately 0.5 to 2 hours, and then cooledto room temperature by gradually lowering the current applied to thehigh-frequency coil.

Step 5: The Al—Fe alloy is taken out of the high-frequency heatingfurnace. In this respect, oxide is likely to reside on the surface(upper face, side face and lower face) of the Al—Fe alloy thus takenout. Therefore, the surface is polished with sand paper to remove theoxide. Alternatively, the Al—Fe alloy is etched with an acid to removethe oxide.

Step 6: The Al—Fe alloy thus free from the oxide is cut into a desiredthickness. In this respect, the thickness is determined according to thediameter and length of the AlN whiskers to be produced. In the case ofproducing, for example, AlN whiskers having a diameter of approximately0.1 to 1 μm and a length of approximately 10 to 50 μm, it is desirablethat the Al—Fe alloy should have a thickness of approximately 0.1 to 5mm. In the case of producing, for example, AlN whiskers having adiameter of approximately 0.5 to 1.5 μm and a length of approximately0.05 to 1 mm, it is desirable that the Al—Fe alloy should have athickness of approximately 0.3 to 2 mm. In the case of producing, forexample, AlN whiskers having a diameter of approximately 1 to 2 μm and alength of approximately 1 mm or larger, it is desirable that the Al—Fealloy should have a thickness of approximately 0.5 to 5 mm. The cuttingmethod is a cutting method using a diamond cutter, a cutting methodusing a laser or a cutting method using wire discharge. Since the oxideis likely to reside on the surface of the Al—Fe alloy even after thecutting, it is preferred that the oxide should be removed by polishingwith a sand paper or by etching with an acid.

The steps described above are steps for producing an Al—Fe alloy byusing a high-frequency heating furnace different from the high-frequencyheating furnace 2 of the AlN whisker production apparatus 1 shown inFIG. 1 or 2. The Al—Fe alloy may be produced using the high-frequencyheating furnace 2 of the AlN whisker production apparatus 1. In short,the Al—Fe alloy may be produced using the same high-frequency heatingfurnace as shown in FIG. 1 or 2, and the AlN whiskers can be producedusing the produced Al—Fe alloy.

(2) Steps for Producing AlN Whiskers

Step 1: The Al—Fe alloy produced by the aforementioned (1) steps forproducing an Al—Fe alloy is loaded on the alumina plate in thehigh-frequency heating furnace 2, and the inside of the high-frequencyheating furnace 2 is vacuumed. In this respect as well, the degree ofvacuum is set to 1 mmHg or lower, preferably 1×10⁻¹ mmHg or lower.

Step 2: After the vacuuming of the inside of the high-frequency heatingfurnace 2, argon gas (purity: 95% or higher, preferably 99% or higher)is introduced to the inside of the high-frequency heating furnace 2 sothat the internal pressure of the high-frequency heating furnace 2 isadjusted to about atmospheric pressure. In this respect as well, it ispreferred that oxygen in the argon gas should be removed with a titaniumgetter, or water vapor should be removed by liquefication in a condensertube cooled with liquid nitrogen.

Step 3: Current is applied to the high-frequency coil 4 to start theheating of the high-frequency heating furnace 2. The high-frequencyheating furnace 2 is heated to a temperature of approximately 1700° C.over approximately 1 to 3 hours. FIG. 12 shows the temperature gradient.

Step 4: After the temperature of the high-frequency heating furnace 2reaches approximately 1700° C. (growth temperature in FIG. 11), the gasto be introduced to the inside of the high-frequency heating furnace 2is changed from argon gas to nitrogen gas. Specifically, the nitrogengas is introduced at a flow rate of, for example, 1 L/min (a nitrogengas atmosphere is formed) to the inside of the high-frequency heatingfurnace 2, and the high-frequency heating furnace 2 is kept at thetemperature of approximately 1700° C. for a predetermined time (growthtime in FIG. 11). The direction in which the nitrogen gas flows is setto a direction parallel to the surface of the Al—Fe alloy. In thiscontext, the term “parallel” means that the nitrogen gas flows from oneend side (from the right side in FIG. 1) toward the other end side(toward the left side in FIG. 1) along the surface of the Al—Fe alloy.In the present embodiment, the case where the nitrogen gas flows isillustrated. Alternatively, the nitrogen gas may be sealed in the insideof the high-frequency heating furnace 2. In this case as well, it ispreferred to remove oxygen in the nitrogen gas or water vapor. On thesurface of the Al—Fe alloy, the Al element (or Al vapor) reacts with thenitrogen gas to produce the core of AlN. Subsequently, on the tip of thecore of AlN, the Al element (or Al vapor) reacts with the nitrogen gasso that AlN grows into a whisker form to produce AlN whiskers.Alternatively, the pressure of the nitrogen gas may be several mmHg toatmospheric pressure or higher. At a higher pressure of the nitrogengas, the growth rate of the AlN whiskers becomes faster. At a lowerpressure of the nitrogen gas, the growth rate of the AlN whiskersbecomes slower. Also, the pressure of the nitrogen gas may be determinedin light of the vapor pressure of Al. The time for which thehigh-frequency heating furnace 2 is kept at the temperature ofapproximately 1700° C. is determined according to the diameter andlength of the AlN whiskers to be produced. In the case of producing, forexample, AlN whiskers having a diameter of approximately 0.1 to 1 μm anda length of approximately 10 to 50 μm, the time is approximately 0.5 to10 hours. In the case of producing, for example, AlN whiskers having adiameter of approximately 0.5 to 1.5 μm and a length of approximately 50to 1 mm, the time is several hours to 20 hours. In the case ofproducing, for example, AlN whiskers having a diameter of approximately1 to 2 μm and a length of approximately 1 mm or larger, the time is 20hours or longer. The high-frequency heating furnace 2 thus kept at thetemperature of approximately 1700° C. for the predetermined time iscooled to room temperature by gradually lowering the current applied tothe high-frequency coil 4.

Step 5: The AlN whiskers are taken out of the high-frequency heatingfurnace 2. These AlN whiskers are grown so as to cover the whole surfaceof the Al—Fe alloy. Then, the AlN whiskers are peeled off from the Al—Fealloy. If the AlN whiskers are difficult to peel off from the Al—Fealloy, the AlN whiskers can be peeled off by using a sharp knife or thelike.

Step 6: The AlN whiskers are stored. The AlN whiskers have a propensityto form ammonia and aluminum hydroxide according to the chemicalequation “AlN+3H₂O→Al(OH)₃+NH₃” through reaction with moisture in theatmosphere. Therefore, it is important that the AlN whiskers taken outof the high-frequency heating furnace 2 are immediately covered with dryair and stored in a depository, such as a desiccator, having anexceedingly small moisture content. Also, it is preferred that such anoperation should be performed in a dry room having very low humidity. Inaddition, the AlN whiskers are dipped in a surface treatment agentrarely reactive with moisture, and dried. For the AlN whiskers, contactwith oxygen is also undesirable. Therefore, it is important that the AlNwhiskers are stored in a depository such as a desiccator as mentionedabove.

In the present embodiment, the reaction temperature is raised toapproximately 1700° C. The reaction temperature can be raised to thetemperature at which the AlN whiskers grow, and can be raised to therange of 1450 to 1800° C. The steps described above employ the AlNwhisker production apparatus 1 shown in FIG. 1 or 2. The AlN whiskerscan also be produced by similar steps using the AlN whisker productionapparatus 31 for mass production shown in FIG. 3. Also, the AlN whiskerscan be produced by similar steps using a continuous tunnel furnace. Inthe case of using the continuous tunnel furnace, a carriage loaded withthe Al—Fe alloy and an alumina plate is disposed in an anterior chamber.In the state where a gate valve which separates the anterior chamberfrom a firing chamber is closed, the anterior chamber is vacuumed, andargon gas is introduced thereto. Then, in the nitrogen atmosphere formedin the firing chamber, the gate valve which separates the anteriorchamber from the firing chamber is opened, and the carriage loaded withthe Al—Fe alloy and an alumina plate is moved from the anterior chamberto the firing chamber. The carriage keeps moving in the inside of thefiring chamber so that the temperature of the firing chamber is raised.The carriage moves in the space of approximately 1700° C. to produce thecore of AlN on the surface of the Al—Fe alloy. Subsequently, on the tipof the core of AlN, the Al element (or Al vapor) reacts with thenitrogen gas so that AlN grows into a whisker form to produce AlNwhiskers. Then, the carriage loaded with the Al—Fe alloy and an aluminaplate is moved from the firing chamber to a posterior chamber and cooledto room temperature.

FIG. 13 shows the aspect in which on the surface of the Al—Fe alloy, theAl element (or Al vapor) reacts with the nitrogen gas to produce thecore of AlN; and subsequently, on the tip of the core of AlN, the Alelement (or Al vapor) reacts with the nitrogen gas so that AlN growsinto a whisker form to produce AlN whiskers. In this case, Fe thatconstitutes the composition with Al is the element satisfying thecondition under which the element forms a compound with neither Al nornitrogen gas or the condition under which the element forms a compoundwith any of Al and nitrogen gas provided that the standard free energyof formation of the compound is larger than that of AlN (the compound isthermodynamically more unstable than AlN). Therefore, the growth of theAlN whiskers is promoted, while the production of particulate AlN issuppressed.

Also, Fe that constitutes the composition with Al is the elementsatisfying the condition under which the interaction energy with Albecomes negative. Therefore, the surface in the axial direction is flat(the surface in the axial direction has very few irregularities). Inshort, the element satisfying the condition under which the interactionenergy with Al becomes negative has a propensity to be easily adsorbedonto Al on the AlN surface. The AlN whisker grown in the <0001>direction has two end faces, one of which is covered with N (N-face) andthe other of which is covered with Al (Al-face). It is generally knownabout the growth of AlN by a sublimation method that the stable growthof the AlN crystal is promoted at the N-face, whereas the growth of theAlN crystal is inhibited at the Al-face, for example, due to theformation of facets such as (10-11). In short, when the AlN crystals areAlN whiskers, the growth of the AlN whisker at the Al-face is inhibitedwhen the element satisfying the condition under which the interactionenergy with Al becomes negative is adsorbed on the Al-face of the AlNwhisker. As a result, the growth of the AlN whisker at the N-face, whichpermits stable growth, can be promoted so that the AlN whisker growsstably toward the direction of the N-face to produce AlN whiskers havingflat surface in the axial direction.

FIG. 14 shows the relationship between the vapor pressure of Al vaporand the vapor pressure of nitrogen gas. As seen from the relationshipshown in FIG. 14, the yield (production efficiency) of the AlN whiskerscan be enhanced provided that the ratio between Al vapor and nitrogengas is 1:1 to 100.

FIG. 15 is an image obtained by photographing the Al—Fe alloy before AlNwhisker production. Each of FIGS. 16 and 17 is an image taken after AlNwhisker production under conditions involving Al (30 at %)-Fe (70 at %)composition, a reaction temperature of 1700° C. and a growth time of 2hours. As is evident from FIGS. 16 and 17, it was confirmed that the AlNwhiskers grow so at to cover the whole surface of the Al—Fe alloy. Eachof FIGS. 18 to 23 is an image taken with a scanning electron microscope.The surface in the axial direction of the AlN whiskers has fewirregularities. Even at a site where having surface irregularities, thedifference between a projection and a depression is several to 10 nm (10nm at a maximum). The reason for such few surface irregularities isthat, as described above, Fe is the element that satisfies the conditionunder which the interaction energy with Al becomes negative and alsosatisfies the condition under which the absolute value of thisinteraction energy is larger than the interaction energy between Al andGe. As shown in FIG. 22, a branched portion was also found. This isbecause the core of AlN is present at some midpoint of the AlN whisker.As shown in FIG. 23, some sites were confirmed to have surfaceirregularities, the degree of which is however only a few out of 10000AlN whiskers, demonstrating that a very large number of AlN whiskershave no surface irregularities.

FIG. 24 shows analysis results of XRD (X-ray diffraction) in the statewhere AlN whiskers were chopped. Most of products were confirmed to beAlN, demonstrating that substances other than AlN (impurities) were notmixed therein. Thus, the yield of the AlN whiskers can be confirmed tobe high.

FIGS. 25 to 30 each show analysis results of EDX in the SEM analysis oftwo sites in the cross section of an arbitrary one of the AlN whiskers.It was confirmed that Fe was not detected. Presumably, the detection ofcarbon was due to background, and the detection of oxygen was due tooxidation that occurred when the AlN whiskers were taken out or thecross section was disrupted. Also, the detection of platinum was due tovapor deposition such as sputtering that occurred when the insulatingmaterial was analyzed.

FIG. 31 is a TEM image of the cross section of an arbitrary one of theAlN whiskers exposed to the atmosphere after production, wherein theimage was taken after a lapse of approximately half a year from theexposure. FIGS. 32, 33 and 34 are mapping images of Al, N and 0,respectively. An oxide layer having a thickness of approximately 8 nm(10 nm at a maximum) was found throughout the surface of the AlNwhisker, whereas no such oxide layer was found in the inside of the AlNwhisker. Thus, the inside of the AlN whisker is presumed to have acrystalline structure of a single crystal that makes the entry of Odifficult. Since the thickness of the oxide layer on the surface of theAlN whisker influences thermal conductivity, reduction in thermalconductivity caused by the influence of the oxide layer having athickness of approximately 8 nm is presumably very small.

FIG. 35 is a mapping image of O of an arbitrary one of the AlN whiskersexposed to the atmosphere after production, wherein the image was takenimmediately after the exposure. In this case as well, an oxide layerhaving a thickness of approximately 10 nm was found throughout thesurface of the AlN whisker, whereas no such oxide layer was found in theinside of the AlN whisker. From FIGS. 34 and 35, it was confirmed thatthe thickness of the oxide layer found in the AlN whisker produced bythe steps of the present invention does not largely differ betweenimmediately after the exposure to the atmosphere and after a lapse ofapproximately half a year from the exposure. Thus, it was confirmed thatwhen the AlN whiskers produced by the steps of the present invention areexposed to the atmosphere after production, an oxide layer having athickness of approximately 10 nm at a maximum is formed immediatelyafter the exposure, and the thickness of the oxide layer formedimmediately after the exposure is not increased even after a lapse ofapproximately half a year (the thickness is rarely changed as comparedwith immediately after the exposure).

Here, the oxide layer will be supplemented. As mentioned above, the AlNwhiskers have a propensity to form ammonia and aluminum hydroxidethrough reaction with moisture in the atmosphere. When the AlN whiskersare used for the purpose of being mixed in a resin as mentioned later,the reaction of the AlN whiskers mixed in the resin with moisturepenetrating the resin has the risk of causing cracks in the resin orcausing the corrosion of a product (e.g., motors, electronic componentsand printed circuit boards) filled with the resin or reduction in thequality of the product. In light of such a situation, it is desirablefor preventing the reaction of the AlN whiskers with moisture that theoxide layer having a moderate film thickness should be formed throughoutthe surface of each AlN whisker. The moderate film thickness is athickness that does not hinder thermal conductivity and insulation,which are the properties of the AlN whiskers. In this respect, the oxidelayer having a thickness of approximately 10 nm is formed by mereexposure to the atmosphere in the steps of the present invention and canensure moisture resistance. This can circumvent the risk of causingproblems with the mixing of the AlN whiskers into the resin.Furthermore, it is unnecessary for securing moisture resistance toreluctantly perform a treatment for forming the oxide layer. Inaddition, if the oxide layer had a thickness over approximately 10 nm(thickness larger than necessary) as the time passed, it would benecessary to perform a treatment for removing the larger-than-necessarythickness of the oxide layer. In the steps of the present invention, thethickness is not increased from approximately 10 nm even after a lapseof at least approximately half a year as compared with immediately afterthe exposure to the atmosphere. Therefore, it is unnecessary to performthe treatment for removing the larger-than-necessary thickness of theoxide layer.

FIG. 36 is a TEM image of the surface of AlN whiskers produced using asolvent of Al—Fe. FIG. 37 is a TEM image of the surface of AlN whiskersproduced using a solvent of Al—Si. A stacking fault was found in thesurface of the AlN whiskers produced using the solvent of Al—Si, whereasneither dislocation nor planar defects were found in the surface of theAlN whiskers produced using the solvent of Al—Fe.

FIG. 38 is a SEM image of the surface of AlN whiskers produced using asolvent of Al—Fe. A curved portion was found in the AlN whiskersproduced using the solvent of Al—Fe. Each of FIGS. 39 and 40 is a TEMimage of the curved portion. Dislocation was found in the curvedportion.

FIGS. 41 and 42 are images obtained by photographing samples differentin shape from the samples shown in FIGS. 15 to 30, before and after AlNwhisker production, respectively. In this case as well, the surface inthe axial direction of the AlN whiskers was confirmed to have fewirregularities. FIG. 43 is an image taken after AlN whisker productionunder conditions involving Al (10 at %)-Fe (90 at %) composition, areaction temperature of 1700° C. and a growth time of 2 hours. Thegrowth of the AlN whiskers was also found when the alloy having Al (10at %)-Fe (90 at %) composition was used. However, its yield wasconfirmed to be low as compared with use of the alloy having Al (30 at%)-Fe (70 at %) composition. FIG. 44 is an image taken after AlN whiskerproduction using an alloy having Al (80 at %)-Fe (20 at %) composition.The growth of the AlN whiskers was also found when the alloy having Al(80 at %)-Fe (20 at %) composition was used. However, the growth wasconfirmed to be not favorable as compared with use of the alloy havingAl (30 at %)-Fe (70 at %) composition. It can be concluded that amongthe samples shown in the present invention, the Al (30 at %)-Fe (70 at%) composition is favorable.

Each of FIGS. 45 to 48 is an image obtained by photographing, with ascanning electron microscope, AlN whiskers produced by the method of the'951 Publication (the method using an alloy having Al—Si—Ti composition)described in, for example, the Summary of the Invention, above, as asubject to be compared with the present invention. The surface in theaxial direction of the AlN whiskers produced by the method of the '951Publication was confirmed to have irregularities, and the differencebetween a projection and a depression is several tens of nm on averageand is approximately 1 μm in a site having a large differencetherebetween. These surface irregularities occurred because, as isevident from FIG. 8 described above, Si that constitutes the compositionwith Al satisfies the condition under which the interaction energy withAl becomes negative, but does not satisfy the condition under which theabsolute value of this interaction energy is larger than the interactionenergy between Al and Ge.

Each of FIGS. 49 to 75 is an image of AlN whiskers produced by changingthe compositional ratio of Al and Fe, the reaction temperature, themetal that constitutes the composition with Al, etc. Each of FIGS. 49 to51 is an image taken after AlN whisker production under conditionsinvolving Al (60 at %)-Fe (40 at %) composition, a reaction temperatureof 1450° C. and a growth time of 2 hours. FIG. 52 is an image takenafter AlN whisker production under conditions involving Al (60 at %)-Fe(40 at %) composition, a reaction temperature of 1500° C. and a growthtime of 2 hours. Each of FIGS. 53 to 55 is an image taken after AlNwhisker production under conditions involving Al (60 at %)-Fe (40 at %)composition, a reaction temperature of 1550° C. and a growth time of 2hours. Each of FIGS. 56 to 58 is an image taken after AlN whiskerproduction under conditions involving Al (60 at %)-Fe (40 at %)composition, a reaction temperature of 1700° C. and a growth time of 2hours. Under these conditions, the surface in the axial direction of theAlN whiskers was also confirmed to have few irregularities. Thus, it wasconfirmed that the Al (60 at %)-Fe (40 at %) composition rarely producesirregularities on the surface in the axial direction of the AlN whiskersunder conditions involving a reaction temperature of 1450 to 1700° C.and a growth time of 2 hours.

Each of FIGS. 59 to 62 is an image taken after AlN whisker productionunder conditions involving Al (30 at %)-Cu (70 at %) composition, areaction temperature of 1700° C. and a growth time of 2 hours. FIG. 63is an image obtained by photographing each metal having Al (30 at %)-Co(70 at %) composition. Each of FIGS. 64 to 67 is an image taken afterAlN whisker production under conditions involving Al (30 at %)-Co (70 at%) composition, a reaction temperature of 1700° C. and a growth time of2 hours. Each of FIGS. 68 and 69 is an image taken after AlN whiskerproduction under conditions involving Al (30 at %)-Ni (70 at %)composition, a reaction temperature of 1700° C. and a growth time of 2hours. Under these conditions, the surface in the axial direction of theAlN whiskers was also confirmed to have few irregularities. Thus, it wasconfirmed that the use of Cu, Co or Ni instead of Fe as the metal thatconstitutes the composition with Al also rarely produces irregularitieson the surface in the axial direction of the AlN whiskers. This isbecause Cu, Co or Ni, as with Fe, is the element satisfying thecondition under which the interaction energy with Al becomes negative.

FIG. 70 is an image obtained by photographing each metal having Al (30at %)-Ge (70 at %) composition. Each of FIGS. 71 to 74 is an image takenafter AlN whisker production under conditions involving Al (30 at %)-Ge(70 at %) composition, a reaction temperature of 1700° C. and a growthtime of 2 hours. FIG. 75 is an image obtained by photographing eachmetal having Al (30 at %)-Sn (70 at %) composition. Each of FIGS. 76 and77 is an image taken after AlN whisker production under conditionsinvolving Al (30 at %)-Sn (70 at %) composition, a reaction temperatureof 1700° C. and a growth time of 2 hours. Under these conditions, unlikeuse of Cu, Co or Ni as the metal that constitutes the composition withAl, AlN whiskers were confirmed to be produced, and however, the surfacein the axial direction of the AlN whiskers was confirmed to haveirregularities. This is because unlike Fe, Cu, Co and Ni, Ge or Sn doesnot satisfy the condition under which the absolute value of theinteraction energy is larger than the interaction energy between Al andGe, though the interaction energy with Al becomes negative.

The AlN whiskers thus produced are mixed into, for example, a resin orfat and oil, and used as a composite member having high thermalconductivity and insulation. FIG. 78 shows the relationship between theamount of the AlN whiskers added to a resin and thermal conductivities.The values were calculated on the basis of the Yamada model according toEtsuro Yamada and Terukazu Ota, “Effective Thermal Conductivity ofDispersed Materials,” Warme and Stoffübertragung 13 (1980), pages 27-37(hereinafter “Yamada”).

In the Yamada model, thermal conductivities are calculated in the statewhere AlN whiskers are present in random directions and can be catalogedin terms of the aspect ratio between the diameter and length of AlN. Inthe case of the same amount of the AlN whiskers added, a larger aspectratio is found to lead to a higher thermal conductivity. The thermalconductivity (250 to 270 W/mk) of the AlN whiskers is calculated on thebasis of the hypothesis that AlN is a single crystal. Approximately 40vol % of a material having an aspect ratio of approximately 100(diameter: approximately 1 μm, length: approximately 100 μm) was mixedinto each polyethylene resin (thermal conductivity: 0.33 to 0.42 W/mk),followed by molding at approximately 250 to 300° C. under pressure. As aresult, the obtained resin mixtures were confirmed to have a thermalconductivity varying from approximately 3 to 10 W/mk. In this case, ifthe AlN whiskers and the resin are poorly mixed (nonuniformly mixed),the thermal conductivity of a portion rich in the resin is close to thevalue of the thermal conductivity of the resin itself while the thermalconductivity of a portion rich in the AlN whiskers is close to the valueof the thermal conductivity of the AlN whiskers themselves. On the otherhand, if the AlN whiskers and the resin are well mixed (uniformlymixed), the thermal conductivity is (converges to) 8 to 9 W/mk. Providedthat the AlN whiskers and the resin can be uniformly mixed, the thermalconductivity less varies, and the average value thereof is slightlyhigher. From the comparison of these results with the Yamada model shownin FIG. 78, it can be concluded that the value was slightly lower thanthe calculation results of the Yamada model, but was not largelydifferent therefrom. Accordingly, the resin mixture having a desiredthermal conductivity can be obtained by changing (adjusting) the aspectratio, added amount, diameter and length of the AlN whiskers. A resinmixture having a higher thermal conductivity can be obtained by using aresin having a high thermal conductivity (e.g., an alignment materialfor main-chain liquid crystalline polyester resins, thermalconductivity: approximately 1 W/mk) and mixing the resin having a highthermal conductivity with the AlN whiskers.

Next, the method for mixing the AlN whiskers into the resin will bedescribed.

Step 1: The desired aspect ratio is determined with reference to theYamada model shown in FIG. 78, and the respective weights of the AlNwhiskers and a powdery resin material are measured. In this respect, itis preferred that the AlN whiskers should be surface-treated.

Step 2: These materials are stirred in a kneading machine. In thisrespect, the internal pressure of the kneading machine can be reduced tothereby efficiently remove air or the like at the contact interfacebetween the resin and the AlN whiskers. Furthermore, a vibration can beapplied to the kneading machine from the outside to thereby efficientlymix the materials.

Step 3: The kneaded materials are vacuum-molded into particles whilepressed at a temperature that partially melts the resin. Subsequently, adesired product is filled with the vacuum-molded materials in a particleform.

Hereinafter, some specific purposes of use of the resin mixturecontaining the AlN whiskers will be illustrated.

FIG. 79 shows an aspect in which the resin mixture (organic compound)containing the AlN whiskers is used as a covering member for an electricwire. An electric wire 101 shown in FIG. 79(a) is configured such that acopper or aluminum wire rod 102 having a square cross section is coveredwith a resin mixture 103. An electric wire 104 shown in FIG. 79(b) isconfigured such that a copper or aluminum wire rod 105 having a circularcross section is covered with a resin mixture 106. Heretofore, enamelpaint has been applied around a wire rod, or scales of mica have beenmixed into enamel paint for heat-resistant wires. By contrast, in thepresent invention, the resin mixture 103 or 106 containing the AlNwhiskers is adopted instead of the enamel paint. Therefore, thermalconductivity better than ever can be expected by the amount (weight) ofmica added smaller than the conventional one. In short, in the case ofapplying the same current, the approach of the present invention cansuppress a rise in the temperature of the wire rod 102 or 105 ascompared with the conventional approach. If the wire rod of the presentinvention and the conventional wire rod have temperature resistantproperties for the same temperature, larger current can be applied tothe wire rod of the present invention.

FIG. 80 shows an aspect in which the resin mixture containing the AlNwhiskers is used as a member for a light bulb. A LED light bulb 111shown in FIG. 80 is configured such that a glass cover 113 is attachedto a resin mixture 112, and two LED elements (114 and 115) areimplemented on a resin substrate 116. The resin mixture 112 contains theAlN whiskers, and the AlN whiskers are also mixed in the resin substrate116. The LED element 114 is supplied with power from a power supplysubstrate 117 via electric wires 118 and 119 to emit light. The LEDelement 115 is supplied with power from the power supply substrate 117via electric wires 120 and 121 to emit light. Upon light emission of theLED elements 114 and 115, heat generated from the LED elements 114 and115 is transferred to the resin mixture 112 via the resin substrate 116and released (dissipated) from the resin mixture 112 into theatmosphere. The surface of the resin mixture 112, i.e., the portion thatcomes into contact with the atmosphere, preferably has a concavo-convexshape that facilitates dissipating heat.

Heretofore, the glass cover has been attached to an aluminum alloy. Bycontrast, in the present invention, the resin mixture 112 containing theAlN whiskers is adopted instead of the aluminum alloy. Therefore, thewhole weight can be reduced. The aluminum alloy is effective in terms ofheat dissipation, but is inferior in insulation performance. Therefore,the aluminum alloy transfers heat via an insulating plate or the like.Hence, thermal conductivity is inhibited at the insulating portion, orthe structure is complicated due to the increased number of components.By contrast, such problems do not arise in the present invention.

FIG. 81 shows an aspect in which the resin mixture containing the AlNwhiskers is used as a member for a motor. A motor 131 shown in FIG. 81is configured such that an iron core material 133 and a coil 134 aredisposed in the inside of a housing 132, and a rotor 138 comprising ashaft 135 and a magnet 136 integrated with an axis 137 for magnetmounting is rotatably supported by a bearing 139. A resin mixture 140containing the AlN whiskers is filled between the rotor 138 and theinner wall of the housing 132. In the case of, for example, a motor forHEV (hybrid electric vehicle), a motor of several tens to severalhundreds of kW is adopted, and approximately 70 to 80% of supplied powerserves as motivity to generate heat of 0 to 30 kW.

Heretofore, an insulating oil has been used for releasing heat generatedin the inside of the motor 131 to the outside, and the motor 131 hasbeen cooled by discharging the insulating oil to the outside of themotor 131, cooling the insulating oil with a heat exchanger, andinjecting (circulating) the cooled insulating oil again to the motor131. By contrast, in the present invention, the resin mixture 140containing the AlN whiskers is adopted instead of the insulating oil.This eliminates the need of preparing a heat exchanger outside the motor131 and releases heat generated in the inside of the motor 131 to theoutside via the housing 132. Such elimination of the need of a heatexchanger can be expected to achieve a compact structure or costreduction. Also, effects equivalent to the scheme of circulating theinsulating oil can be expected provided that a resin mixture obtained bymixing, for example, AlN whiskers having a thermal conductivity ofapproximately 20 W/mk into a polyimide resin (decomposition temperature:500° C. or higher) can be filled thereto and a radiation fin can beattached to the housing 132. The magnet 136 generally tends to exhibitdecrease in magnetic force at Curie temperature or higher. For example,the Curie temperature of a NdFe magnet is 330° C., and the NdFe magnethas the properties of deteriorating its magnetic force at thistemperature or higher. By contrast, the deterioration in the magneticforce of the NdFe magnet can be prevented even at or around its Curietemperature 330° C. by adopting a resin mixture obtained by mixing, forexample, AlN whiskers having a thermal conductivity of approximately 20W/mk, for example, into a polyimide resin.

FIG. 82 shows an aspect in which the resin mixture containing the AlNwhiskers is used as a member for an electronic component. An electroniccomponent 141 shown in FIG. 82(a) is configured such that, for example,Si or GaAs semiconductor element 143 implemented on a substrate 142 ismolded with a resin mixture 144. An electronic component 145 shown inFIG. 82(b) is configured such that a semiconductor element 147implemented on a substrate 146 is molded, together with a portion of awire 148 and a pin 149, with a resin mixture 150. The electroniccomponent 145 is produced by the following steps: the semiconductorelement 147 is implemented onto the substrate 146, and the semiconductorelement 147 and the pin 149 are connected through the wire 148(electrically connected by wire bonding). In this state where thesemiconductor element 147 and the pin 149 are connected through the wire148, the melted resin mixture 150 is injected into the mold while themold is heated. In this operation, the resin mixture 150 is injectedinto the whole mold in vacuum so as not to form space (so as not toleave air bubbles). Then, the resin mixture 150 is cured by cooling andthen taken out of the mold, together with the semiconductor element 147,etc. In this case, it is preferred that the AlN whiskers used shouldhave a length having no chance to cut the wire 148, preferably, a lengthof for example, several to 100 μm. In the case of energization with thesame power, use of a resin mixture 150 having a thermal conductivity ofapproximately 20 W/mk can drastically lower the temperature of thesemiconductor element as compared with use of a conventional epoxy resin(e.g., thermal conductivity: approximately 0.1 to 0.5 W/mk).

FIG. 83 also shows an aspect in which the resin mixture containing theAlN whiskers is used as a member for an electronic component. Anelectronic component 151 shown in FIG. 83(a) is a power card for use inHEV or the like and is an instrument having the function of convertingvoltage or the function of converting direct current to alternatingcurrent. An IGBT element 153 is implemented on a substrate 152. The IGBTelement 153 and an electrode 154 are electrically connected through awire 155. The electronic component 151 is produced by the followingsteps: the IGBT element 153 is implemented onto the substrate 152, andthe IGBT element 153 and the electrode 154 are connected through thewire 155 (electrically connected by wire bonding). In this state wherethe IGBT element 153 and the electrode 154 are connected through thewire 155, a melted resin mixture 156 is injected into the mold while themold is heated. In this operation as well, the resin mixture 156 isinjected into the whole mold in vacuum so as not to form space (so asnot to leave air bubbles). Then, the resin mixture 156 is cured bycooling and then taken out of the mold, together with the IGBT element153, etc. In this case, it is also preferred that the AlN whiskers usedshould have a length having no chance to cut the wire 155, preferably, alength of for example, several to 100 μm. Then, a radiation fin 158having many blades 157 is jointed to the resin mixture 156.

Heretofore, an IGBT element having copper-plated surface has beenmounted on a substrate, and the IGBT element and an electrode have beenconnected by wire bonding and fixed with an epoxy resin. In thisrespect, the epoxy resin has been prevented from remaining on thecopper-plated surface; silicone grease having a high thermalconductivity (thermal conductivity: approximately 1 to 3 W/mk) has beenapplied to the copper-plated surface; ceramic plate (thermalconductivity: approximately 30 to 36 W/mk) and silicon nitride plate(thermal conductivity: approximately 30 to 80 W/mk) materials have beenattached thereon; silicone grease has been further applied thereto; andan aluminum heat exchanger has been attached thereto. By contrast, theapproach of the present invention can eliminate the need of siliconegrease, a ceramic plate, a silicon nitride plate, copper plating, etc.,and can simplify the structure. When a resin mixture 156 containing theAlN whiskers and having a thermal conductivity of, for example, 20 W/mkis used at a thickness of 0.3 to 0.5 mm, the heat discharge isequivalent to or higher than that of the configuration using a siliconnitride plate. In the electronic component 151 shown in FIG. 83(a), theradiation fin 158 is jointed on only one side (on the lower side in FIG.83). The radiation fin 158 may be jointed on both sides (on the lowerand upper sides in FIG. 83), as in an electronic component 161 shown inFIG. 83(b).

FIG. 84 shows an aspect in which the resin mixture containing the AlNwhiskers is used as a member for a printed circuit board. A printedcircuit board 171 shown in FIG. 84(a) is composed of a resin mixture 174containing the AlN whiskers as a simple substance. A printed circuitboard 172 shown in FIG. 84(b) is configured such that a resin mixture174 is jointed to a conductor part 175 made of copper foil or aluminumfoil. A printed circuit board (multilayer substrate) 173 shown in FIG.84(c) is configured such that resin mixtures 174 are jointed to multiplelayers of conductor parts 175.

Heretofore, a printed circuit board has been formed by mixing glassfibers into an epoxy resin. In this case, heat dissipation is hardlytaken into consideration and is approximately 0.01 to 0.1 W/mk in termsof a thermal conductivity, though insulation is secured. By contrast, inthe present invention, the resin mixture 174 containing the AlN whiskerscan be used to produce a printed circuit board having a thermalconductivity of, for example, 20 W/mk. Since many holes are opened inthe printed circuit boards 171 to 173, it is preferred to fillrelatively short AlN whiskers therein. Such filling of the relativelyshort AlN whiskers can prevent reduction in thermal conductivity whilefacilitating opening holes. The printed circuit board 173, which is amultilayer substrate, is produced by the following steps: the AlNwhiskers are introduced into an epoxy resin, which is then prepared intoa preliminary molded article in a thin sheet form or plate form. Copperfoil or a copper plate (or aluminum foil or an aluminum plate) isjointed onto the preliminary molded article, and unnecessary portions onthe copper foil or the copper plate are removed. A plurality of suchlayers are stacked, then cured by pressing under heating at apredetermined temperature, and cooled. A printed circuit board 181 shownin FIG. 85 is configured such that aluminum foil 182, a resin mixture183 containing the AlN whiskers and a copper plate 184 are jointed.

FIG. 86 shows an aspect in which the resin mixture containing the AlNwhiskers is used as a member for a sheet. A sheet 191 is configured suchthat a thermally conductive acrylic sheet 192, a resin mixture 193containing the AlN whiskers and a film liner are jointed. Heretofore, athermally conductive low-hardness acrylic layer has been jointed below athermally conductive acrylic resin (non-viscous), and a film filler hasbeen further jointed therebelow. By contrast, in the present invention,a thermally conductive low-hardness acrylic resin is used, and the AlNwhiskers are mixed into the thermally conductive low-hardness acrylicresin. The length of the AlN whiskers is preferably several to 100 μm,and the amount of the AlN whiskers added is preferably, for example,approximately 40 wt % or lower. AlN whiskers added in an amount largerthan necessary significantly reduce viscosity.

FIG. 87 shows an aspect in which the AlN whiskers are contained in a fatand oil mixture (organic compound). Grease 201 (fat and oil mixture) isa silicone resin 202 in which the AlN whiskers are mixed. The length ofthe AlN whiskers to be mixed into the silicone resin is preferablyseveral to several tens of μm. Longer AlN whiskers are unsuitablebecause these AlN whiskers increase a coating thickness when appliedthereto. Since the mixing of the AlN whiskers enhances viscosity, it isdifficult to mix the AlN whiskers in a large amount. Approximately 40 wt% of the AlN whiskers can achieve a thermal conductivity ofapproximately 10 W/mk.

FIG. 88 shows an aspect in which the AlN whiskers are mixed in paper.Such paper is produced by steps given below. First, cellulose fibersserving as a raw material for paper 211 are dissolved in water. Then,the surface of the AlN whiskers is coated with a film that preventsreaction with water. Then, the AlN whiskers are added at approximatelyseveral to 50% (vol %) with respect to the amount of the cellulosefibers in the solution containing the cellulose fibers dissolved inwater, and an organic paste (paste material such as methylcellulose)serving as a binder is added thereto at approximately several to 5%(which is a ratio to the total amount of the cellulose fibers and theAlN whiskers). Then, the solution is uniformly mixed by stirring andstrained through a net-like filter to decrease the water content as muchas possible. In this case, the paper 211 is thickened with increase inthe number of runs to strain the solution. Therefore, the desiredthickness is achieved by adjusting the number of runs to strain thesolution. Then, the material is taken out of the filter, made intopaper, dried and then rewound in a roll form. The paper 211 in which theAlN whiskers are mixed is used for purposes that require insulation andrequire thermal conductivity. The paper 211 is used for a purpose, forexample, capacitor paper for condensers. Also, the paper 211 is used fora purpose as flame-retardant paper.

FIG. 89 shows an aspect in which the AlN whiskers are mixed in a fiber.Such a fiber is produced by steps given below. First, a material servingas a raw material for a fiber (e.g., an organic chemical fiber) 221 isdissolved in water or an organic solvent. Then, the surface of the AlNwhiskers is coated with a film that prevents reaction with water. Then,the AlN whiskers are added at approximately several to 50% (vol %) withrespect to the amount of the material serving as a raw material for thefiber in the solution containing the material dissolved in water or anorganic solvent, and an organic paste (paste material such asmethylcellulose) serving as a binder is added thereto at approximatelyseveral to 5% (which is a ratio to the total amount of the material andthe AlN whiskers). Then, the solution is uniformly mixed by stirring andstrained through a net-like filter to decrease the water content as muchas possible. In this case as well, the fiber 221 is thickened withincrease in the number of runs to strain the solution. Therefore, thedesired thickness is achieved by adjusting the number of runs to strainthe solution. The material is taken out of the filter, made into cloth,dried and then rewound in a roll form. The fiber 221 in which the AlNwhiskers are mixed is used for purposes that require flame retardanceand heat dissipation, for example, curtain materials, by bonding orintertwining fibers 221 by mechanical or scientific action and forming anonwoven fabric.

FIG. 90 shows another aspect in which the AlN whiskers are mixed in afiber. Such a fiber is produced by steps given below. First, an organicmaterial serving as a raw material for a fiber 231 is dissolved in anorganic solvent. Then, the surface of the AlN whiskers is coated with afilm that prevents reaction with water. Then, the AlN whiskers are addedat approximately several to 50% (vol %) with respect to the amount ofthe organic material for the fiber 231 in the solution containing theorganic material dissolved in an organic solvent, followed by spinningfrom the solution. In this way, the AlN whiskers are mixed in theorganic material. Then, the fiber 231 obtained by spinning is dried andthen rewound. The fiber 231 in which the AlN whiskers are mixed is usedfor purposes such as clothes having good thermal conduction by weavingspun thread into cloth. The fiber 231 can also be used in curtains,carpets, and the like to thereby produce flame retardance and thermalconductivity. Also, the fiber 231 is used in electric heater carpetmaterials. The resulting materials are excellent in insulation andthermal conductivity and can permit uniform heating, leading to savingin electricity cost. Alternatively, the fiber 231 may be used in gloveshaving thermal conductivity and heat resistance, or may be used inprotective covers (tubes) for thermocouples because of exerting heatresistance, insulation and thermal conductivity when mixed with aninorganic fiber (alumina fiber, mullite fiber or silica fiber). As shownin FIG. 91, several fibers 241 may be bundled and twisted to formtwisted thread, which can in turn be used in, for example, clotheshaving good thermal conductivity or rope or code having good dissipationof frictional heat.

The AlN whiskers can also be applied to the purpose of enhancing coolingeffects. For example, cooling water (thermal conductivity of water:0.569 W/mk at 0° C. and 0.685 W/mk at 100° C.) is used in the cooling ofvehicle engines. AlN whiskers surface-treated to be waterproof can beadded into cooling water to thereby enhance the thermal conductivity ofthe cooling water. In this case, the amount of the AlN whiskers added ispreferably an amount that does not impair flowability, and ispreferably, for example, 60% or smaller by volume. It is also preferredto add AlN whiskers having a diameter of approximately 0.1 to 1 μm and alength of approximately 0.5 to 100 μm so as not to separate the AlNwhiskers from the water.

The addition of the AlN whiskers into a cooling solvent for organicmaterials is also effective. For example, the AlN whiskers can be addedinto a lubricating oil (thermal conductivity: 0.148 W/mk at 0° C. and0.135 W/mk at 120° C.) to thereby enhance the thermal conductivity ofthe lubricating oil. In this case as well, the amount of the AlNwhiskers added is preferably an amount that does not impair flowability,and is preferably, for example, 60% or smaller by volume. It is alsopreferred to add AlN whiskers having a diameter of approximately 0.1 to1 μm and a length of approximately 0.5 to 100 μm so as not to separatethe AlN whiskers from the lubricating oil. Also, the AlN whiskers may beadded into silicone oil (thermal conductivity: 0.134 W/mk at 25° C.) orspindle oil (thermal conductivity: 0.144 W/mk at 20° C. and 0.138 W/mkat 120° C.).

The mixing of the AlN whiskers described above for the resin mixture orthe fat and oil mixture is not limited to the mixing of only the AlNwhiskers with no surface irregularities obtained by the presentinvention. The AlN whiskers with no surface irregularities andconventional AlN whiskers with surface irregularities may be used incombination for the mixing. A resin mixture containing 20 vol % of theAlN whiskers with no surface irregularities and 20 vol % of the AlNwhiskers with surface irregularities was confirmed to produce betterthermal conductivity than that of a resin mixture containing 40 vol % ofthe AlN whiskers with surface irregularities. Also, a resin mixturecontaining 20 vol % of the AlN whiskers with no surface irregularities,10 vol % of the AlN whiskers with surface irregularities and 10 vol % ofparticulate AlN was confirmed to produce thermal conductivity nearlyequal to that of the resin mixture containing 40 vol % of the AlNwhiskers with surface irregularities. Furthermore, a resin mixturecontaining 30 vol % of the AlN whiskers with no surface irregularitiesand 10 vol % of particulate AlN was also confirmed to produce thermalconductivity nearly equal to that of the resin mixture containing 40 vol% of the AlN whiskers with surface irregularities.

As described above, according to the present embodiment, an element thatsatisfies the condition under which the element forms a compound withneither Al nor nitrogen gas or the condition under which the elementforms a compound with any of Al and nitrogen gas provided that thestandard free energy of formation of the compound is larger than that ofAlN (the compound is thermodynamically more unstable than AlN) is usedas the element that constitutes the composition with Al. As a result,the element that constitutes the composition with Al for the AlN whiskerproduction is not limited to Si and Ti, and the AlN whiskers can beproduced using an element other than Si and Ti. In the presentembodiment, it was confirmed that AlN whiskers are produced by using Fe,Co, Cu, Ge or Sn as the element that constitutes the composition withAl. The AlN whiskers can also presumably be produced by using Li, Mg, V,Cr, Mn, Ga or Sr, which satisfies the condition (i).

The element used satisfies the condition under which the interactionenergy with Al becomes negative and also satisfies the condition underwhich the absolute value of this interaction energy is larger than theinteraction energy between Al and Ge. As a result, AlN whiskers havingflat surface (surface with very few irregularities) in the axialdirection can be produced. In the present embodiment, it was confirmedthat the surface in the axial direction is flat by using Fe, Co or Cu asthe element that constitutes the composition with Al. The surface in theaxial direction is also presumably flat by using Cr or Sr, whichsatisfies the condition (ii).

In the case of mixing the AlN whiskers having no irregularities on thesurface in the axial direction into a resin or into fat and oil, the AlNwhiskers easily come in surface contact with each other without comingin point contact and can thus provide a resin mixture or a fat and oilmixture excellent in thermal conductivity and insulation. The resinmixture or the fat and oil mixture containing the AlN whiskers can beused in a portion that requires thermal conductivity and insulation in aproduct, for example, an electric wire, a light bulb, a motor, anelectronic component, a sheet, paper or a fiber to thereby enhance thequality of the product.

The present invention is not intended to be limited by the embodimentsdescribed above and can be modified or expanded as mentioned below.

The present invention is not limited by the method for producing AlNwhiskers and can also be applied to a method for producing a thin filmor a bulk of AlN.

The present invention is not limited by the method for producing AlNwhiskers by using a binary composition such as Al—Fe or Al—Cu and canalso be applied to a method for producing AlN whiskers by using aternary or higher-order composition, for example, an Al—Fe—Cu ternarycomposition.

The product obtained using the resin mixture containing the AlN crystalsmay be a product other than an electric wire, a light bulb, a motor, anelectronic component, a sheet, paper and a fiber. Specifically, theresin mixture containing the AlN crystals can be applied to a portionthat requires thermal conductivity and insulation and thereby applied toany product.

1. A method for producing AlN crystals, comprising: using at least oneelement, excluding Si, that satisfies a condition under which theelement forms a compound with neither Al nor N at a predeterminedreaction temperature or a condition under which the element forms acompound with any of Al and N provided that the standard free energy offormation of the compound is larger than that of AlN; melting acomposition containing at least Al and the element; and reacting the Alvapor with nitrogen gas at a predetermined reaction temperature toproduce whiskers consisting of AlN crystals from the alloy containing atleast Al and the element.
 2. The method for producing AlN crystalsaccording to claim 1, wherein the element used is any of Li, Mg, V, Cr,Mn, Fe, Co, Ni, Cu, Ga, Ge, Sr and Sn.
 3. The method for producing AlNcrystals according to claim 1, wherein the element used is an elementthat satisfies a condition under which the interaction energy with Albecomes negative and also satisfies a condition under which the absolutevalue of this interaction energy is larger than the interaction energybetween Al and Ge.
 4. The method for producing AlN crystals according toclaim 3, wherein the element used is any of Li, Cr, Fe, Co, Ni, Cu andSr. 5.-9. (canceled)
 10. AlN crystals, wherein the crystals arewhiskers, and the difference between a projection and a depression onthe surface is smaller than 10 nm at a maximum. 11.-12. (canceled) 13.An organic compound comprising AlN crystals, wherein the organiccompound is constituted by mixing therein crystals according to claim10.
 14. An electric wire obtained using an organic compound comprisingAlN crystals according to claim 13 as at least any of a thermallyconductive member and an insulating member.
 15. A light bulb obtainedusing an organic compound comprising AlN crystals according to claim 13as at least any of a thermally conductive member and an insulatingmember.
 16. A motor obtained using an organic compound comprising AlNcrystals according to claim 13 as at least any of a thermally conductivemember and an insulating member.
 17. An electronic component obtainedusing an organic compound comprising AlN crystals according to claim 13as at least any of a thermally conductive member and an insulatingmember.
 18. A printed circuit board obtained using an organic compoundcomprising AlN crystals according to claim 13 as at least any of athermally conductive member and an insulating member.
 19. A sheetobtained using an organic compound comprising AlN crystals according toclaim 13 as at least any of a thermally conductive member and aninsulating member.
 20. Paper obtained using an organic compoundcomprising AlN crystals according to claim 13 as at least any of athermally conductive member and an insulating member.
 21. A fiberobtained using an organic compound comprising AlN crystals according toclaim 13 as at least any of a thermally conductive member and aninsulating member.
 22. An AlN crystal production apparatus, comprising:a core tube; and a unit for heating, wherein the production is carriedout by using at least one element, excluding Si, that satisfies acondition under which the element forms a compound with neither Al nor Nat a predetermined reaction temperature or a condition under which theelement forms a compound with any of Al and N provided that the standardfree energy of formation of the compound is larger than that of AlN;melting a composition containing at least Al and the element by heatingwith the unit for heating in the inside of the core tube; and reactingthe Al vapor with nitrogen gas at a predetermined reaction temperatureto produce whiskers consisting of AlN crystals from the alloy containingat least Al and the element.